RE: [TCML] PFC Question (again)

["Mark R Dunn" <teslamark@xxxxxxxxxxxxx>]

All:

I agree with Gary, but here is some math to back it up.
Dave used a 700 uF cap so that is 3.79 ohms Reactance (Capacitive)
So he will draw 30+ amps across the line with the capacitors connected and
the coil not running.  This is all reactive power.

When he fires up the coil, the Reactance (Inductive) from his tranny begins
to offset the capacitive Reactance and the current drops.

Here is a worked example(my SISG coil):

Take a 2000 VA coil that pulls 1000 watts of real power (This example is an
MOT based coil).
Then the Reactive power = sqrt(2000^2 - 1000^2) = 1732 VA
and the Reactance (Inductive) = 120^2/1732 = 8.3 ohms
(120 volt power supply)
So the PFC = 1/(2*Pi*60Hz*8.3 ohms) = 320uF

Now without PFC the coil pulls 17 amps at the line (2000/120)
With PFC the coil pulls 8 amps (1000/120)
BUT turn the coil off and leave the PFC connected 
and it pulls 15 Amps !!! (120/8.3)
But this is all reactive power and real power is zero.
When the coil turns on the current drops back to 8 amps!!

So as Gary said when the PT is unloaded it has little Reactance (Inductive),
but the large cap presents a large reactance (capacitive).

Mark

>Let me see if I understand this.  You added a 700 uF PFC cap to the primary
>side of the unloaded PT.

>I don't think this is surprising.  Since the PT is unloaded, it should
>present only a small reactive load, but the huge PFC cap presents a huge
>reactive load.  Maybe if the PT was capacitively loaded, driving a TC, it
>might cancel?  Can you tell that I'm talking just a little bit through my
>hat ;-) ?

>Regards, Gary Lau
>MA, USA


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